The invention pertains to the field of multi-step pushbutton switches for electronic devices. The invention addresses the problem of providing a pushbutton switch which has multiple switch positions which can be reliably sensed by an operator of the switch.
A "pushbutton switch" is conventionally understood in the field to describe a switching mechanism which employs a button or similar structure which is depressed to select among successive switch positions. A pushbutton switch having more than one switch position is known as a multistep switch.
In multistep switches, it is conventional to provide structure in the switch which allows the individual operating the switch to distinguish among the successive switch positions by means of a counterpressure which increases with successive switch positions. For example, in a pushbutton switch having first, second and third switch positions, three successive counterpressures may correspond to the three switch positions. The first position may be sensed as a first counterpressure having a first magnitude generated by a "stop". The button may thereafter be further depressed to attain a second switch position. Upon attaining the second position, a second counterpressure of a second magnitude which is greater than that of the first counterpressure will be sensed. By applying still further pressure, the third position may be attained. A third counterpressure of a third magnitude which is greater than that of the second counterpressure will be sensed when the third switch position is attained. Typically, where the selected position corresponds to depression of the switch to its maximum depth, the counterpressure is generated by a "fixed stop" which obstructs further depression of the button.
Pushbutton switches of the type described above are found, for example, in cameras having electronic control systems. The various switch positions may typically correspond to modes of operation of the camera. For example, when the switch is depressed to the first position, the camera may respond by measuring the light level and focal distance of a subject to be photographed. Further depression of the switch to the second position may then cause the camera to photograph the subject in accordance with the measurements previously taken.
Two conventional multistep pushbutton switches are known. The first conventional switch uses a switching rod which may be depressed against the force of a restoring spring. Two bolts extend from the switching rod in the axial direction of the switching rod. The degree of extension of each bolt is adjustable. The switching rod and the bolts extend toward two spring biased electrical contacts which correspond to the bolts. As the switching rod is depressed, a first bolt will make contact with its corresponding spring biased electrical contact. This contact may, for example, connect the bolt to ground potential. When this contact is made the switch has attained the first position. The operator will sense the increased counterpressure generated by the spring biased electrical contact and will recognize that the switch is in the first position. At a certain point of further depression the second bolt will make contact with its corresponding spring biased electrical contact. When the second bolt makes contact, the switch attains the second position. The operator will sense the increased counterpressure generated by both spring biased electrical contacts and will recognize that the switch is in the second position.
In pushbutton switches of the type described above, the counterpressures corresponding to each switch position must be differentiable to the operator from the friction forces of the switching rod and the counterpressure generated by the restoring spring. In practice, it has been found that the counterpressure generated by the second electrical contact is often sensed only weakly by the operator. Therefore it is generally not possible to indicate more than two switch positions by means of cumulative counterpressures. If a further switch position is desired to be provided, this is conventionally done by providing a slide contact on the switching rod which makes contact with an associated electrical contact at approximately the point at which further movement of the switching rod is prevented through contact with a fixed end stop. Accordingly, the operator is not able to precisely sense the point at which the third switching state is attained.
Pushbutton switches of the above type present difficulties in installation and use. The design of the switching rod is complex, and the bolts and their associated contacts must be precisely positioned in order for the switch to operate properly. In addition, all of the individual parts of this type of switch must be installed separately and then aligned. This makes both installation and repair a time consuming and complex procedure.
The pushbutton switch disclosed in German Patent Application DE 43 04 304 A1 is representative of the second conventional type of pushbutton switch. This type of switch employs a switching rod that is depressed to make contact with a cantilevered dome-shaped diaphragm. The diaphragm is positioned with its face adjacent a circuit board to which is mounted an annular switching contact and a central contact located within the anular contact. When the switching rod is pressed down against the dome-shaped diaphragm, the edge of the dome-shaped diaphragm initially makes contact with the annular switching contact, thereby achieving a first position and producing an increased counterpressure which can be sensed by the operator. If further pressure is applied, the dome of the diaphragm flexes inward and contacts the central contact. Contact with the central contact achieves a second switch position and can be sensed by the operator as a fixed stop. Switches of this type are of limited use in that it is not possible to provide more than two switch positions.